Describe the issue:

In NumPy main, when I do an inplace build, I can no longer import NumPy. I get

Traceback (most recent call last):
  File "./numpy/core/__init__.py", line 23, in <module>
    from . import multiarray
  File "./numpy/core/multiarray.py", line 10, in <module>
    from . import overrides
  File "./numpy/core/overrides.py", line 6, in <module>
    from numpy.core._multiarray_umath import (
ModuleNotFoundError: No module named 'numpy.core._multiarray_umath'

I bisected it to 2ae7aeb.

There are several errors in the build log, which I've included here https://gist.github.com/asmeurer/40754d1a4429449e1d558e003caa1860.

There seem to be some issues with the build scripts with C++. Perhaps they were never tested with clang?

This is currently blocking my ability to properly test my PR #20066.

Reproduce the code example:

git clean -dfX
python setup.py build_ext --inplace
python -c 'import numpy'

Error message:

No response

NumPy/Python version information:

This is the latest NumPy main. I am using Python 3.9.2, on macOS.